The present invention is directed, in general, to location systems and, more specifically, to a system and method for locating a probe tip (or any other object) relative to a locator made up of markers. The arrangement of markers in the locator is such that bit encoding errors that may occur as the probe tip or other object scans the locator are reduced or eliminated.
It has always been desirable to test integrated circuits (ICs) to determine whether they have been manufactured properly. While scanning electron microscopes have been available for such testing, they require a sample to be sectioned and thus destroyed. Nondestructive testing is preferred by far. Stylus, or scanning probe, microscopes offer an opportunity to perform such nondestructive testing.
Unfortunately, the device and interconnect structures that make up today""s ICs have become so small that, when attempting to probe an IC sample with a scanning probe microscope, it has become difficult to determine where exactly the tip of the microscope""s probe is located relative to the sample. Ignorant of the exact relative location of the probe tip, signals that the probe tip returns as it scans structures in the sample lose their meaning and become uninterpretable. Lacking a proper interpretation, the scanned structures cannot be evaluated, and vital information that could have been used to improve the manufacturing processes employed to fabricate the IC is lost. As a consequence, IC yields drop and manufacturing time and cost rise.
Scanning probe microscopes are able to locate their probe tips relative to a sample with a degree of uncertainty (perhaps a few microns). Unfortunately, that degree of uncertainty has become unacceptably large relative to the size of the structures on the IC. To address this problem, probe tip locators have been developed to determine the location of the probe tip more precisely, i.e., reduce the degree of uncertainty to an acceptable level.
A probe tip locator is itself a structure that the probe can scan. The locator is placed on the stage of the microscope, along with the sample, at a location that is a known distance and direction from the sample. The locator is laid out such that a scan of the locator by the probe tip provides the information required to locate the probe tip relative to the locator. Having located the probe tip relative to the locator, and knowing the relative distance and direction between the locator and the sample, the location of the probe tip relative to the sample can be determined. As a result, the probe tip can be accurately placed on the sample, and the structures in the sample meaningfully analyzed.
Prior art probe tip locators typically used a single, large structure consisting of a first reference line parallel to a reference axis (for example, the y-axis) and a second reference line intersecting the first at a known angle. When a probe tip scanned the structure, it returned signals indicating the location of the first line (thereby fixing the locator with respect to, continuing the example, the x-axis) and the location of the second line. Knowing these locations and the speed at which the probe tip scanned the structure, the distance between the first and second lines is determinable. Since only one y-axis location could evidence such distance (and given that the angle of intersection is known), the locator is therefore theoretically fixed with respect to the y-axis. The probe tip is so located and sample analysis can begin.
By their nature, the first and second lines gradually separate as they depart their intersection point. This is unfortunate, since the probe tip is thus required to scan an ever-increasing distance to cross both lines as the line the probe tip scans (the xe2x80x9cscan linexe2x80x9d) is separated from the intersecting point. Since scanning probe microscopes have limits on the length of the scan line their probe tips are able to traverse, and given that the probe tip must cross both lines, prior art probe tip locators were limited in terms of their size.
What is needed in the art is a probe tip locator that can allow the probe tip of a scanning probe microscope to be located to an acceptably small location within an acceptably large locator, such that manufacturing and testing of ICs (and, more broadly, monolithic circuits) are improved.
More generally, what is needed in the art is an improved way of locating any object relative to a locator based entirely on a knowledge of the layout of the locator and what the object encounters as the object traverses the locator.
As stated above, the present invention is related to U.S. patent application Ser. No. 09/196,827, now U.S. Pat. No. 6,178,653. That application sets forth two embodiments of a probe tip locator that overcome the deficiencies of the prior art probe tip locator discussed above.
The first embodiment employs a set of lines (or, more generically, xe2x80x9clocation markersxe2x80x9d) that form bit fields and neighboring intersecting lines that together cooperate to locate a probe tip relative to the locator. The second embodiment employs multiple sets of location markers that form bit fields that cooperate to locate a probe tip relative to the locator.
The markers are arranged in the bit fields to encode unique addresses for their respective locations. As a probe tip scans an unknown location on the locator, it produces signals that indicate the unknown location""s address, revealing the location. Depending upon the number of bits in the bit fields and limited only by the resolution of the lines and the length of the path that the scanning probe microscope is able to accommodate, the probe tip can be located to an acceptably small location within an acceptably large locator.
Both embodiments happen to assign sequential addresses to adjacent locations. As advantageous as both embodiments are, this orderly assignment of addresses has caused a problem. As a probe tip scans along a scan line, it traverses an area roughly equaling its width multiplied by the length of the scan line. This area is hereinafter called a xe2x80x9cscanpath.xe2x80x9d Since the probe tip has a finite width, its scanpath has a corresponding finite width, which means that a probe tip may, on occasion, traverse a scanpath that straddles two adjacent locations on the probe tip locator.
Should a probe tip""s scanpath happen to straddle two adjacent locations, location markers from the two locations can interfere to cause the probe tip to produce a signal representing an address that corresponds to neither one of the two adjacent locations. A bit encoding error of this type is best described as xe2x80x9caliasing.xe2x80x9d
Aliasing produces an erroneous determination of the probe tip""s location relative to the locator. This corrupts any further calculations or analysis that depends on knowledge of the location and ultimately prevents a proper interpretation of sample structures. Accordingly, an objective of the present invention is to reduce or eliminate the chance that aliasing can occur.
In the attainment of this objective, the present invention provides a probe tip locator for, and method of, use in determining a location of a probe tip relative to the probe tip locator. The probe tip locator includes sets of discrete location markers in which numbers and positions of the location markers in each of the sets are employable uniquely to identify corresponding specific locations on the probe tip locator. The sets are distributed about the probe tip locator to avoid unbalanced partial encroachments into both sides of a scanpath of the probe tip by location markers in sets normally adjacent the scanpath (xe2x80x9cdouble-sidedxe2x80x9d unbalanced partial encroachments). This prevents an erroneous determination of location caused by unbalanced partial encroachments of the location markers into both sides of the scanpath as the probe tip traverses the scanpath.
The present invention therefore introduces the broad concept of arranging the sets of location markers that constitute a probe tip locator such that the sets do not encroach upon the scanpath and cause erroneous location indications. This arranging can be done in at least two alternative ways.
A first way calls for the sets to be spaced apart from one another by at least a width of the scanpath. The addresses of the locations then can be ordered in any manner, including sequentially.
A second way calls for the addresses to be ordered such that opposing state transitions (both 0xe2x86x921 transitions and 1xe2x86x920 transitions) between corresponding bit fields of locations normally adjacent all possible scanpaths are avoided. Thus ordered, further spacing is unnecessary. State transitions give rise to unbalanced partial encroachments; opposing state transitions give rise to the double-sided unbalanced partial encroachments that the present invention seeks to avoid.
The present invention enjoys substantial utility in that errors are reduced, increasing the reliability and decreasing the cost of manufacturing and testing monolithic circuits.
In one embodiment of the present invention, the location is a Cartesian location. The location may alternatively be polar or of any other conventional or later-discovered coordinate system.
In one embodiment of the present invention, the probe tip locator further includes reference markers distributed about the probe tip locator at predetermined ordinal locations thereon. The reference markers, while not necessary to the present invention, are employable to differentiate sets from one another in the ordinal direction (the direction of probe tip travel in the embodiment to be illustrated and described).
In one embodiment of the present invention, the location markers are bit fields. In an embodiment to be illustrated and described, the probe tip returns a binary signal representing at least a partial presence or a complete absence of the bit fields. Of course, those skilled in the art will perceive that many different types of markers, including those of varying dimension or shape, may be employed to advantage and remain within the broad scope of the present invention.
In one embodiment of the present invention, the sets of discrete location markers are first sets of discrete location markers, the probe tip locator further comprising second sets of discrete location markers cooperate with the first sets of location markers uniquely to identify two-dimensional specific locations on the probe tip locator. Thus, the present invention is not limited to a one-dimensional locator; it fully encompasses multidimensional locators wherein the number of sets cooperating to describe a particular location equals the number of dimensions included in the location.
In one embodiment of the present invention, the scanpath is linear. This need not be the case, however. The probe tip needs only to encounter such markers as necessary to determine its location, no matter how those markers are positioned relative to one another.
In one embodiment of the present invention, discrete markers are embodied in a structure on a monolithic substrate. Thus, the probe tip senses the relief. However, the probe tip may sense any physical characteristic associated with the location markers and is not limited to sensing relief.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.